The present invention generally relates to apparatus and methods for measuring the temperature of liquid dispensing apparatus and, more specifically, the temperature of an electric dispensing valve.
Electric viscous liquid dispensers have been developed for dispensing applications requiring precise placement of a viscous liquid. Typically, an electric dispenser is coupled to a manifold which is connected to a supply of viscous thermoplastic liquid such as hot melt adhesive. Collectively, the electric dispenser and the manifold serve as a liquid dispensing unit. Viscous thermoplastic liquids, such as hot melt adhesive, solidify at lower temperatures. Therefore, they must remain heated as they flow through the manifold and the electric dispenser. Accordingly, the manifold may incorporate a heating device to maintain the hot melt adhesive in the manifold and in the electric dispenser at the appropriate application temperature.
A temperature sensing device incorporated within the manifold effectively monitors the temperature of the hot melt adhesive in the manifold. However, the electric dispenser usually does not include a temperature sensing device as it is assumed that the temperature of the hot melt adhesive in the manifold is equivalent to the temperature of the hot melt adhesive in the electric dispenser. For pneumatically actuated dispensers, this assumption is acceptable as the operation of a pneumatically actuated gun does not generate appreciable heat which would tend to increase the temperature of the hot melt adhesive flowing therethrough. For electric dispensers, however, this assumption may not be accurate.
Activating the electric coil of an electric dispenser can generate considerable additional heat. Excessively elevated temperature can have undesirable effects on the characteristics of the hot melt adhesive and the valve performance. For instance, the increase in temperature may degrade the integrity of the adhesive, making accurate dispensing difficult if not impossible. In addition, the increase in temperature may affect the valve performance by altering the valve""s response time. Also, the temperature of the electric coil may become so elevated that it may exceed the melting point of the insulation around the coil wire.
Consequently, measuring the temperature of the hot melt adhesive within the manifold may not provide sufficient temperature information for the electric dispenser and the hot melt adhesive flowing therethrough. The industry therefore needs improvements related to monitoring the temperature of the electric dispenser, instead of only the manifold. Monitoring the temperature of the electric dispenser will better protect against overheating the viscous liquid and the electric coil during operation of the liquid dispensing unit.
The present invention provides a heated liquid dispensing unit that overcomes the drawbacks of past units as described above. The invention is particulary directed to overcoming problems associated with dispensing units incorporating an electrically actuated dispenser and dispensing thermoplastic liquids. In accordance with the principles of the present invention, the liquid dispensing unit includes an electrically operated dispensing valve which is adapted to couple to a source of heated thermoplastic liquid and is selectively operative to dispense the heated thermoplastic liquid. The dispensing valve has an external surface. A temperature sensing device is operatively associated with the dispensing valve and has a sensing element contacting the external surface of the dispensing valve. The temperature sensing device is operative to measure the temperature of the external surface and thereby indirectly measure the temperature of the heated liquid therein. The sensing element may be a resistance temperature detector (RTD), a thermocouple, or any other suitable temperature sensor.
In another embodiment, the liquid dispensing unit includes a manifold which has a liquid passageway adapted to connect to a source of heated thermoplastic liquid. An electrically operated dispensing valve is coupled to the manifold. The dispensing valve includes an external surface and a liquid passageway which is in fluid communication with the liquid passageway of the manifold. The dispensing valve is selectively operative to dispense the heated thermoplastic liquid. The liquid dispensing unit further includes a temperature sensing device operatively associated with the dispensing valve. The temperature sensing device has a sensing element contacting the external surface of the dispensing valve to measure the temperature of that external surface and thereby indirectly measure the temperature of the heated thermoplastic liquid within the dispensing valve.
In another aspect of the invention, the manifold also has a first mating surface. The electrically operated dispensing valve has a second mating surface which is coupled to the first mating surface of the manifold. The dispensing valve has a liquid passageway which is in fluid communication with the liquid passageway of the manifold. The dispensing valve is selectively operative to dispense the heated thermoplastic liquid. The liquid dispensing unit further includes a temperature sensing device affixed to the manifold. The temperature sensing device includes an elongated member with an end which extends through the manifold and sensing element affixed to that end. A biasing member is operatively coupled to the elongated member to bias the sensing element into positive engagement with the second mating surface of the dispensing valve to measure the temperature of that mating surface and thereby indirectly measure the temperature of the heated thermoplastic liquid within the dispensing valve. One main advantage of this embodiment is that removal of the dispensing valve does not require disruption of the temperature sensing device or its wiring.
The present invention also includes a method for sensing the temperature of a heated thermoplastic liquid flowing through a dispensing valve coupled to a manifold and having an electrically operated actuator and an external surface. The method includes directing the heated thermoplastic liquid from the manifold to the dispensing valve and heating the external surface of the dispensing valve with heat generated by the actuator and heat dissipating from the heated thermoplastic liquid. The temperature of the liquid is then measured by sensing the temperature of the external surface of the dispensing valve.
Various additional advantages and features of the invention will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description taken in conjunction with the accompanying drawings.